Liquid crystal display based front and rear projection systems show great potential for High Definition (HD) and three dimensional video applications due to their superior resolution. Contrast is considered an important performance specification of a projection system, as it ultimately influences the number of true gray levels and the color fidelity. A challenge in such projection systems is to achieve acceptable system contrast despite subtle depolarization effects within the optical modulation system.
In projection displays using LCoS or other LC panels, there is a need to compensate residual, OFF-state panel retardance to ensure sufficient contrast performance, because such residual in-plane retardance applied to incident optical rays can cause polarization mixing and lead to OFF-state leakage. In the case of large screen televisions based on LC panel projection, this leakage manifests itself as a bright dark-state and one that is often colored. When displaying dark video content, such leakage is very obvious and undesirable. Removing residual OFF-state retardance of the LC panels, or at least its adverse affect, can be achieved by introducing birefringent elements in front of the panel, which was described by U.S. Patent Publication No. US 2003/0128320, to Xiang-Dong Mi, and by M. Robinson in commonly-assigned U.S. patent application Ser. No. 10/908,671.
In general, compensators act first to remove residual in-plane OFF-state retardance of the panel, and second to reduce OFF-state light leakage due to the out-of-plane retardance which relates to field-of-view (FOV) performance of the LC layer. Removing the in-plane retardance is important since it corresponds to the extent to which the LC molecules are not aligned normal to the substrate or not balanced in their orientations when projected onto the panel plane. The substantial out-of-plane retardance alters the polarization state of off-axis rays, and acts to reduce the panel's field-of-view and in non-collimated systems leads again to OFF-state leakage. To get the high contrast demanded of current commercial video projection systems, both in-plane and out-of-plane compensation is desired.